1. Field of the Invention
The present invention relates to wireless communication. In particular, the present invention relates to wireless communication using low-power mobile terminals.
2. Discussion of the Related Art
In a xe2x80x9c2-way pagingxe2x80x9d network, in addition to the messaging application, a user can be provided convenience access to information services, such as weather reports, traffic conditions, stock quotes, and even access to wide area data networks, such as the internet. In one such wireless communication network 100, shown in FIG. 1, transmitter 102 of a network control unit 104 broadcasts messages to mobile terminals (e.g. mobile units 101a-101c) in its service area. Each mobile unit, which is uniquely identified by an identification code, includes a receiver, a low-power transmitter, a display panel (e.g. 4-line liquid crystal display), and a simple keyboard. In wireless communication network 100, numerous receivers (e.g. 103a-103c) are distributed in the service area for receiving transmissions from the nearby mobile terminals, so as to minimize the power requirements of these mobile terminals. Receivers 103a-103c relays the received transmissions of the mobile units to network control unit. Power conservation in mobile terminals is important to achieve a reasonable service battery life before battery replacement or recharging is necessary. One disadvantage of this system is that, at any given time, only one message can be sent from the network control center. The latencies that can occur because of messages queuing at the network control center renders this wireless communication network unsuitable for transaction processing applications (e.g. fund transfers between bank accounts).
An improvement upon the above communication network is achieved by providing, instead of receivers 103a-103c, bidirectional base stations which are capable of communication locally with nearby mobile terminals. An example of a wireless communication network supporting bidirectional communication with local base stations is described in a copending patent application (xe2x80x9cCopending Applicationxe2x80x9d), entitled xe2x80x9cWireless Network Access Schemexe2x80x9d by Weijia Wang, Ser. No. 08/542,860, filed on Oct. 13, 1995, attorney. Copending Application is hereby incorporated by reference in its entirety to provide background information regarding such 2-way paging networks. In this system, because a mobile terminal communicates primarily with its closest base station, system capacity is much greater than the system illustrated is FIG. 1, owing to concurrent and non-interfering use of the spectrum by mobile terminals and base stations not within range of each other.
In general, the range of a transmitter increases with its power but decreases with its data rate. For example, if a given range can be covered by a half-watt transmitter transmitting at 800 baud, to allow transmitting at 9600 baud over the same range, the power requirement on the transmitter may rise to 5.5 watts. Because of the low-power requirement and the need to maintain a reasonable service range, mobile terminals in the prior art communicate at around 1200-2400 baud. To increase channel capacity, i.e. to provide a higher data rate, either the range of the mobile terminal must decrease or a higher power transmitter must be provided to the mobile terminals. If the range of the mobile terminal is decreased, the number of base stations necessary to service the mobile terminals in a given service area must be increased. To achieve a higher data rate, the cost of system hardware increases rapidly.
According to the present invention, a wireless communication network includes (a) a network control center; (b) numerous mobile terminals each receiving messages from the network control center at a first data rate and transmitting messages to the network control center at a second data rate, the first data rate being higher than the second data rate; and (c) numerous base stations coupled to the network control center for receiving messages from the mobile terminals and relaying the messages to the network control center. The base stations are also used to relay messages from the network control center to the mobile terminals. Under this wireless communication network, capacity of the network can be increased without requiring a shorter range, or a higher power transmission capability, in the mobile terminals.
In one embodiment of the wireless communication network, the mobile terminals transmit and receive messages at separate time slots, so that full-duplex circuits need not be provided in the mobile terminals. In another embodiment of the wireless communication network, full-duplex circuits are provided in the base stations to allow transmission and reception of messages simultaneously. In that embodiment of the wireless communication network, separate frequencies are used in transmission and reception of messages in the mobile stations. In one example, the transmission and reception frequencies are both approximately a predetermined carrier frequency, but separated by the intermediate frequency used to modulate the baseband signal.
The present invention is better understood upon consideration of the detailed description below and the accompanying drawings.